The hydrogen atom transfer reactions that underlie peroxide-initiated modifications of polyolefin architectures and ambient-temperature air oxidations are examined through a combination of experiments and quantum chemical calculations on model hydrocarbons. The regioselectivity of H-atom from pentane, 2,4-dimethylpentane, and 2,2,4,4-tetramethylpentane to t-BuO center dot generated from tert-butyl hyponitrite at 25 degrees C was quantified by trapping of the alkyl radicals with a UV-active nitroxide. The data show that hydrocarbon reactivity toward t-BuO center dot does follow neither well-established trends in homolytic C-H bond dissociation energy (tertiary > secondary > primary) nor trends in the susceptibility of polyolefins to autoxidation. These experiments, together with quantum chemical calculations conducted at the CBS-QB3 level of theory, reveal the importance of entropic effects in these exergonic H-atom transfer processes. In contrast, the endergonic nature of H-atom abstraction by t-BuO center dot produces conventional hydrocarbon reactivity patterns, as enthalpic contributions to transition-state energies are dominant. These findings bring clarity to the structure/reactivity relationships of polyolefins in the field of radical chemistry.